The present invention relates to power takeoffs for industrial vehicles and in particular it refers to a device for engaging such power takeoffs, able to be actuated electrically by means of a solenoid.
As is well known, the power takeoff is a mechanical device with gears, connectable to the transmission of industrial vehicles and suitable for transmitting power from the gear to an external use, in particular to a rotary hydraulic pump which transforms mechanical power into hydraulic power to be supplied to a hydraulic system for multiple applications.
FIG. 1 shows an example of a power takeoff according to the state of the art.
The power takeoff is mainly constituted by:
a main body or box 1 (usually made of cast iron or aluminium) which constitutes the outer container of all the other mechanical elements and serves as fixed connection to the gear;
a main gear 2 which is engaged with the gear inside the gearbox and which transfers motion from the transmission gear to other gears inside the power takeoff;
one or more auxiliary gears 3 of the power takeoff;
an output shaft 4 which transmits motion from the internal gears of the power takeoff to the external device (usually a pump);
an engaging device which comprises an engaging assembly 5 that allows to transfer motion and hence power only when it is moved in an active position, called “engaged takeoff position”, and actuating means 7 to move said engaging assembly to said active position; and
an engagement indicator 6 which sends a signal to a receiver (usually, an electronic unit or an indicator light or a sound signalling device) only when the power takeoff is in the engaged position.
Depending on the type of the actuating means of the engaging assembly, currently used engaging devices can be subdivided into the following categories:                Mechanical engaging device: the power takeoff shifts from the inactive state to the active state through the displacement of a lever positioned in the cab of the vehicle and connected to the power takeoff by means of a cable inserted in a sheath and connected directly to the engaging assembly of the power takeoff itself. The motion of the lever is transmitted rigidly to the engaging assembly internal to the power takeoff, allowing it to shift from the inactive position to the active position.        Pneumatic engaging device (see FIG. 1): the engaging assembly inside the power takeoff is actuated by a single effect pneumatic piston contained in the power takeoff. The return of the power takeoff to the inactive position takes place by spring action. The device is actuated by compressed air available on the vehicle (air tank of the auxiliary services of the vehicle itself or dedicated compressor). The device is operated from the vehicle cab by means of manual pneumatic distributor or by the use of a switch controlling an electrical valve.        Vacuum engaging device: the system exploits the same concept as the pneumatic version using the pressure difference between environment and the power brake or power steering circuit of the vehicles. The system can be single or double effect. The system is operated from the vehicle cab by the use of a switch controlling one or more electrical valves (depending on whether the system is double or single effect).        Hydraulic engaging device: the system is wholly comparable to the pneumatic system with the sole difference that the fluid which transmits the force to the engaging system is oil and not air.        Hydraulic or pneumatic clutch engaging device: the pressurised fluid (oil or air) is introduced into a piston inside the power takeoff which acts against a system of clutch disks which transmit motion between the gears inside the takeoff and its output shaft. The system is operated from the vehicle cab by a control switch.        Electrical engaging device: the engaging assembly is moved by an electric motor, controlled by an electronic unit, through a worm screw-sliding block or other converter of rotary motion into linear motion. The system is operated from the vehicle cab by a control switch. An example of such a device is described in U.S. Pat. No. 4,651,852.        Electro-hydraulic engaging device: the engaging assembly is moved by a pressurised fluid created by a mini electrical pump incorporate in the power takeoff. The system is operated from the vehicle cab by a control switch.        Single effect solenoid engaging device: the engaging assembly is moved by a solenoid connected externally to the power takeoff and which transfers motion directly and rigidly or indirectly to the system inside the power takeoff. The return of the power takeoff to the inactive position takes place by spring action. The solenoid is constituted by a large single coil driven with a high current and hence with considerable use of energy and consequent overheating of the solenoid. Alternatively, the solenoid is constituted by two or more windings which are supplied power in different phases during engaging and maintaining with electrical, electromechanical or electronic systems. The latter device is described, for example, in U.S. Pat. No. 5,287,939.        Double effect solenoid engaging device: the engaging assembly is moved by a solenoid connected externally to the power takeoff and which transfers motion directly and rigidly or indirectly to the system inside the power takeoff. The return of the power takeoff to the inactive position takes place by solenoid action. The solenoids are constituted by a large single coil driven with a high current and hence with considerable use of energy and consequent overheating of the solenoids. Alternatively, the solenoids are constituted by two or more windings which are supplied power in different phases during engaging and maintaining with electrical, electro-mechanical or electronic systems. Devices of this kind are described for example in U.S. Pat. Nos. 3,577,789 and 4,669,562.        
A problem with all the above mentioned engaging devices is evident in the engaging phase, which is better described below and which can be called “tooth against tooth jamming”. In all the above mentioned devices, in fact, such jamming leads to the unsuccessful completion of the engaging phase and then to the necessary repetition of this phase by the operator. This leads, above other things, to a partial damaging of the gear teeth for every unsuccessful attempt and then, over a long period, to a failure of the power takeoff.
Another problem of the prior art devices is that, in order to furnish information on the real engaging state of the takeoff (which is basic for the takeoff interface towards the electronic units for the control of the vehicle automatic gear), it is necessary to use and install on the takeoff an end-stroke sensor, called “engagement signaler”. For mounting such signaler on the power takeoff it is necessary to provide for a hole that communicates with the outside. Therefore, possible oil leakage points are introduced, with the consequence of a possible breakdown of the gears on which the takeoff is installed.
Possible malfunctions of said engagement signaler can also lead to wrong information to the gears electronic control unit, and therefore to damaging actions on said gears.
A further problem with the prior art devices is the impossibility of furnishing information about the temperature of the gear on which the power takeoff is mounted. Light vehicle gears are not provided with a system for controlling the temperature and, in case they work with lack of internal lubricant, they quickly get damaged.